SOME FUNDAMENTALS OF HEREDITY IN HORSES
Until very recent times, the general principle that“ like begets like” was the only recognized concept of heredity. That the application of this principle over a long period of time has been effective in modifying animal types in the direction of selection is evident from a comparison of present-day types of animals within each class of livestock. Thus, the speed of representatives of the modern Thoroughbred— coupled with their general lithe, angular build and nervous temperament is in sharp contrast to the slow, easy gaits and docility of the Tennessee Walking Horse. Yet there is good and substantial evidence to indicate that both breeds descended from a common ancestry. Because of the diverstiy of genes carried by the original parent stock, it has been possible, through selection, to evolve with two distinct breeds— one highly adapted to fast running at extended distances and the other to a slow, ambling gait. Also, through selection accompanied by planned matings, this same parent stock has been altered into horses especially adept as hunters, jumpers, stock horses, polo mounts, three- and five-gaited park hacks, harness race horses, etc.
The following is a condensation of a few of the known pertinent facts relative to the genetics of horses:
The Gene is the unit of heredity. In the body cells of horses there are a number of pairs of bundles, called chromosomes. In turn, the chromosomes carry pairs of minute particles, called genes, which are the basic hereditary material. The nucleus of each body cell of horses contains 32 pairs of chromosomes, or a total of 64; whereas there are thousands of pairs of genes. When a sex cell( a sperm or an egg) is formed, only one chromosome and one gene of each pair goes into it. Then when mating and fertilization occur the single chromosomes from the germ cell of each parent unite to form new pairs, and the chromosomes and genes are again present in duplicate in the body cells of the embryo. Thus, with all possible combinations of 32 pairs of chromosomes( the species number in horses) and the genes that they bear, it is not strange that full sisters( except identical twms from a single egg split after fertilization) are so different. Actually, wc can marvel that they bear as much resemblance to each other as they do.
Because of this situation, the mating of i mare with a line track record to a stallion that on the average transmits relatively good performance will not always produce a foal of a merit equal to its parer ts. The foal could be markedly poorer than the parents or. happily,
28 it could in some cases be better than either parent.
Simple and multiple gene inheritance occur in h as in all animals. In simple gene inheritance, only one n^’ of genes is involved; thus, a pair of genes may be resno ible for color of hair in horses. However, most charact— such as speed— are due to many genes; hence they a called multiple-gene characters. For most characters [ n
or more pairs of genes are involved. Growth rate in [ L \ for example, is affected by( 1) appetite and feed con’ sumption,( 2) the proportion of the feed eaten that is ab ' sorbed, and( 3) the use to which the nutrients are put-J whether they arc used for growth or fattening. Because multiple characters show all manner of graduation— from high to low performance, they are sometimes referred to as quantitive traits. Thus, quantitative inheritance refers to the degree to which a characteristic is inherited; for example, all race horses can run and all inherit some ability to run, but it is the degree to which they inherit the ability that is important.
Dominant and recessive factors exist in horses. Some genes have the ability to prevent or mask the expression of others, with the result that the genetic makeup 0f such animals cannot be recognized with accuracy. This is called dominance. The gene that is masked is recessive. Thus, black is dominant to chestnut; hence when a pure black stallion is crossed on a chestnut mare, all of the offspring will be black. The resulting black is not genotypically pure, however; it is Bb, where B stands for the dominant black and b for the recessive chestnut. This black animal will produce germ cells carrying black and chestnut genes in equal proportion. Then if an F1 stallion is crossed on Fx mares, the F2 population will, on the average, consist of three blacks to one chestnut. The chestnut— being a recessive— will be pure for color; that is, the mating of two chestnut horses will produce, according to the most authoritative work, chestnut offspring, which is the situaion in the Suffolk breed of draft horses where all animals of the breed are chestnuts. Of the three black, in the F2, however, only one is pure for black( with the genetic constitution BB). The other two will be Bb in genetic constitution, and will produce germ cells carrying B and b in equal proportion.
As can be readily understood, dominance often makes it difficult to identify and discard all animals carrying an undesirable recessive factor. Also, in some cases dominance is neither complete nor absent, but incomplete or partial and expressed in a variety of ways. The best known case of this type in horses is the Palomino color.
Heredity and environment in quantitative traits function in horses, just as they do in all animals. Generally horse trainers believe that heredity is most impoiant, whereas horse owners believe that environment partic urly training, is most important— especially if they ose a race. Actually; maximum development of characters^ economic importance— growth, body form, speed, etc. cannot be achieved unless there are optimum condition of nutrition and management.
The problem of the horse breeder is that of selecting
the very best animals available genetically— these 0 ^ parents of the next generation. The fact that on y 30 percent of the observed variation may be due to c * lty, and that environmental differences can produce leading variations, makes mistakes in the selectio breeding animals inevitable. a Sex is determined by chromosomes. The lTialC l? cs) pair of similar chromosomes( called x chromoso
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VOICE of the Tennessee Walking Hors